In a typical spatial light modulator (SLM) projection system, such as a digital micromirror device (DMD) based system, there are two types of illumination problems that can lower the system contrast; (1) a portion of the input illumination can overlap some of the projected light from the ON pixels, lowering the system contrast and (2) unwanted off-state and flat-state light, can diffract and scatter into the projected image illumination path, further reducing the image contrast. This unwanted diffracted light can result from scattering of light off various surfaces, such as mirrors transitioning from ON-to-OFF or OFF-to-ON states, device package, device structure, window, and prisms.
It is known from experimental and analytical data that input illumination rays at angles closest to the projection axis contribute most to diffraction contrast problems. Attempts have been made to increase the illumination angle further away from the projection axis to address the diffraction problem, but this typically has the unacceptable result of reduced image brightness or non-uniformity.
A significant portion of the diffracted light is at an angle that gets collected by the projection optics and as a result is projected on to the display screen as undesirable background light. Diffracted light is most evident on the screen when the SLM is in the OFF-state, lighting up the screen where the image is supposed to be very dark. Even with most of the pixels in the ON-state, diffracted and scattered light will brighten portions of the image area that is intended to be dark, thereby lowering the image contrast.
Current approaches to this problem have been to increase the angle of illumination by 2-4 degrees. For example, for a DMD with mirrors that tilt ±10-degrees, the input illumination angle is normally 20-degrees. Changing this angle to 22-degrees makes the approach angle shallower and thereby reduces the diffracted light rays that enter the projection aperture. However, there is a compromise of an additional 2-degrees of light on the far side of the cone that misses the aperture and is lost, thereby reducing the brightness of the image. Attempts to recover this lost light have been made by increasing the illumination aperture, but this results in additional diffraction and overlapping cones of projection and flat-state light, which in turn tends to further decrease the contrast.
In addition, several approaches have been tried to manage tile OFF-state and flat-state light by the use of baffles or apertures in the projection light path. But apertures that pass the projection light also pass any OFF-state and flat-state light that spatially and angularly overlaps the aperture region. Other methods use a TIR prism surface to pass the projection rays, but this requires three prism elements; i.e., one to TIR the illumination rays, one to TIR the flat-state and OFF-state rays, and a third one to add a wedge of glass to compensate for the thickness variation (back working distance) in the projection path. Other approaches direct the unwanted light into an optical heat sink (light trap), often reflecting off various total internal reflective (TIR) surfaces along the optical path, but do this too far along the optical path to prevent overlapping with the desired projection light.
FIG. 1 is a block diagram of a typical single-SLM projection system, which would typically have contrast problems due to overlapping of illumination with projected light and scattering of diffractive light into the projected light beam. This system is comprised of a lamp assembly 100 (illumination source), a condenser lens 102, a rotating color filter wheel assembly 104, an integrator rod 106, relay lenses 108-112, a TIR prism assembly 114, a SLM (DMD) 118, and a projection lens 120.
In operation, light from the lamp assembly 100 is focused to a small spot at the surface of the color filter wheel 104 by means of the condenser lens 102. Sequential color light (R-G-B) coming through the color wheel 104 is integrated by the integrator rod 106 and coupled into a set of relay lenses, made up of a first 108, second 110, and third 112 lens, which shapes the color light beam to fit the optical aperture of a TIR prism 114. The sequential color light being coupled into the TIR prism strikes a first TIR surface 116 at an angle greater than the critical angle of the surface and reflects off the surface on to the surface of a DMD 118. Modulated light is reflected from the ON-mirrors of the DMD 118 back through the TIR prism assembly 114, this time striking the TIR surface 116 at an angle less than the critical angle of the surface, and therefore passes through the surface, out of the prism assembly 114 into the projection lens 120.
The conventional TIR prism of FIG. 1 enables the angular separation of illumination and projected light, as described in FIG. 2. In this ideal case, where there is no scattering or overlapping of light, then foul non-overlapping cones of light exist; i.e., the full illumination cone 204, an ON-state cone 214, a flat-state cone 220, and an OFF-state cone 224. In a bi-stable DMD, light from the lamp 200 striking the surface of the DMD can be reflected from the ON-state 206, the OFF-state 210, and flat-state 208 mirrors in transition or from other flat surfaces in the device. For a DMD having a ±10° tilt angle and −20° illumination angle 202, the center of the projection cone 212 is located at 0°, the center of the light cone 218 reflected from flat-surfaces is located at +20°, and the center of the light cone 222 reflected from OFF-pixel is located at +40°. For example, a DMD with ±10° tilt angle and an f/#3 optics (f/#3=F 228÷D 230=3 for 9.5° beam tilt) using BK7 glass (with n=1.518), produces four non-overlapping cones each having a 19-degree solid cone angle, thereby leaving a ½-degree separation between cones. Light 212 from the projection cone 214 is coupled into a projection lens 216, while unwanted light 218 from the flat-surface cone 220 and unwanted light 222 from the OFF-pixel cone 224 is absorbed in an optical heat sink 226 and discarded. However, in the real world where diffraction and scattering of light exists, some of the unwanted light makes its way into the projection light cone 214 and lowers the contrast of the image. This shows up on the projection screen with background and dark areas of the image not being as dark as they should be, resulting in a “washed out” image. The present invention is about preventing this unwanted light from contaminating the projection cone so that a high-contrast image is projected.
What is needed is an optical approach that (1) removes any portion of the input illumination that might overlap with the projection image along the projection axis, before the illumination reaches the SLM, (2) removes unwanted light from OFF-pixels and flat-surfaces immediately as the light is reflected from the SLM, and (3) recovers the brightness level lost from removing a portion of the input illumination. The present invention accomplishes these goals, using angular separation in a TIR prism, to remove a portion of the input light, before the light can illuminate the SLM and by filtering unwanted light from OFF-state pixels and flat-surfaces away from the projection path, thereby projecting a clean image with high contrast on to the screen. This approach truncates all cones from all SLM states proportionally, thereby allowing the optical aperture to be increased without generating new overlapping cones of output light.